动态创建表，为什么第一次可以，第二次老是出错？（关闭窗口在打开）

Question

unit Unit3;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, DBCtrls, Grids, DBGrids, ComCtrls, TabNotBk, Menus,
  XPMenu, StdCtrls, Buttons, DB, DBTables, Mask;type
  TForm3 = class(TForm)
    MainMenu1: TMainMenu;
    N1: TMenuItem;
    N2: TMenuItem;
    N3: TMenuItem;
    N4: TMenuItem;
    N5: TMenuItem;
    TabbedNotebook1: TTabbedNotebook;
    DBNavigator1: TDBNavigator;
    DBGrid1: TDBGrid;
    BitBtn1: TBitBtn;
    Button1: TButton;
    Label1: TLabel;
    DBEdit1: TDBEdit;
    Label2: TLabel;
    DBEdit2: TDBEdit;
    Label3: TLabel;
    DBEdit3: TDBEdit;
    Label4: TLabel;
    DBEdit4: TDBEdit;
    Label5: TLabel;
    DBEdit5: TDBEdit;
    Label6: TLabel;
    DBEdit6: TDBEdit;
    Label7: TLabel;
    DBEdit7: TDBEdit;
    Label8: TLabel;
    DBEdit8: TDBEdit;
    BitBtn2: TBitBtn;
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure FormShow(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure BitBtn2Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;var
  Form3: TForm3;
   table:TTable;
source:Tdatasource;
implementationuses Unit2, myunit;{$R *.dfm}procedure TForm3.FormClose(Sender: TObject; var Action: TCloseAction);
begin
 if assigned(form3) then table.Free;
if assigned(source) then source.Free;
form3.Free;
form3:=nil;
end;procedure TForm3.FormShow(Sender: TObject);begin
if not assigned(table) then
begin
table:=TTable.Create(self);
table.DatabaseName:=exefilename;
table.TableName:='工资生成表';
table.Open;
source:=tdatasource.Create(self);
source.DataSet:=table;
dbgrid1.DataSource:=source;
DBNavigator1.DataSource:=source;
dbedit1.DataSource:=source;
dbedit2.DataSource:=source;
 dbedit3.DataSource:=source;
 dbedit4.DataSource:=source;
 dbedit5.DataSource:=source;
 dbedit6.DataSource:=source;
 dbedit7.DataSource:=source;
 dbedit8.DataSource:=source;
end;
end;procedure TForm3.Button1Click(Sender: TObject);
begin
table.Edit;
table['应发费用']:=table['基本工资']+table['计件工资']+table['加班费']+table['奖金']+table['其他']-table['应扣费用'];
table.Post;
table.Append;
end;procedure TForm3.BitBtn2Click(Sender: TObject);
begin
if assigned(form3) then table.Free;
if assigned(source) then source.Free;
form3.Release;
form3:=nil;
end;end.